Objective lens driving device and optical disc apparatus using the same

ABSTRACT

It is an object to perform a thin and compact design of the objective lens driving device. In order to achieve the above object, optical beam is passed through a space surrounded by a tracking coil and a focusing coil which are adjacent to each other and fixed in a holder. Further, it is another object to position the objective lens with high precision in an objective lens driving device for switching a plurality of objective lenses in accordance with the type of an optical disc. In order to achieve this object, magnetic substances which are radially extended with the sliding shaft at the center are provided in the lens holder, and the magnetic substances are located within a plane perpendicular to the plane constituting the magnetic gap and at the outside of the magnetic gap. Still further, in an objective lens driving device it is another object to prevent the occurrence of positional displacement of the objective lens due to resistance force of current supply means such as FPC. To achieve this object, a first objective lens and a second objective lens are arranged so that the objective lens arrangement angle at which the first and second objective lenses are arranged with respect to the shaft center of the sliding shaft is displaced, by a predetermined amount, from the magnetic circuit arrangement angle at which the magnetic substances and the magnets are arranged with respect to the shaft center of the sliding shaft.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an objective lens driving deviceand an optical disc apparatus using the objective lens driving device.

[0003] 2. Description of Related Art

[0004] One type of objective lens driving device for use in an opticaldisc apparatus is disclosed in Japanese Laid-open Utility ModelApplication No. Hei-2-35330.

[0005]FIG. 9 shows the constitution of the objective lens drivingdevice, FIG. 10 is a cross-sectional view of the objective lens drivingdevice which is taken along a line G-G of FIG. 9, and FIG. 11 is a viewof FIG. 9 which is taken along the direction of an arrow H.

[0006] An objective lens 7 is provided in a lens holder 3, which issupported and guided by a sliding shaft 6 which is slidably androtatably disposed substantially in parallel to the optical axisdirection of the objective lens 7. A focusing coil 5 is wound around theouter peripheral surface of the lens holder 3, and tracking coils 4 areprovided at predetermined intervals in the peripheral direction on theouter peripheral portion of the lens holder 3. A magnetic circuitcomprising a magnet 2, an outer yoke 1 and an inner yoke 9 is providedto generate magnetic flux so that the magnetic flux traverses thetracking coils 4 and the focusing coil 5. The tracking coils 4 and thefocusing coil 5 are supplied with current from current supply means suchas a flexible printed circuit (hereinafter referred to as “FPC”).

[0007] During a focus control operation, the electromagnetic force actsin the direction of the sliding shaft 6 by causing the current to flowinto the focusing coil 5 in the forward or reverse direction to move theobjective lens 7 in the optical axis direction in accordance with theplane vibration of the recording surface of the optical disc, wherebythe spot of the optical beam 8 can follow the recording surface of theoptical disc. Further, during a tracking control operation, theelectromagnetic force acts as a rotational couple of forces on the outerperiphery of the sliding shaft 6 by causing the current to flow intoeach of the tracking coils 4 in the forward or reverse direction,whereby the spot of the optical beam 8 can follow the eccentricity ofthe track of the optical disc.

[0008] Next, the construction of a magnetic spring in a conventionalobjective driving device will be described by exemplifying an objectivelens driving device disclosed in the official gazette for KOKOKU No.Hei-7-31814.

[0009]FIGS. 12 and 13 show an objective lens driving device having oneobjective lens which is used in a conventional CD apparatus or CD-ROMapparatus, wherein FIG. 12 is a diagram showing the constitution of theobjective lens driving device and FIG. 13 is a cross-sectional view ofthe objective lens driving device which is taken along a line K-K ofFIG. 12.

[0010] In FIGS. 12 and 13, reference numeral 21 represents an objectivelens for focusing an optical beam, reference numeral 22 represents alens holder, reference numeral 28-1, 28-2 represents a focusing coil,reference numeral 29-1, 29-2 represents a tracking coil, referencenumeral 25 represents a sliding shaft, reference numeral 23 representsan inner yoke, reference numeral 24 represents an outer yoke, referencenumeral 26-1, 26-2 represents a focusing magnet, reference numeral 27-1,27-2 represents a tracking magnet, and reference numeral 30-1, 30-2represents a magnetic substance for positioning the objective lens.

[0011] The focusing magnet 26-1, 26-2 is magnetized to have bipolarityin the height direction, and the tracking magnet 27-1, 27-2 ismagnetized to have bipolarity in the peripheral direction. The magneticdensity distribution in the peripheral direction in the neighborhood ofthe magnetic substance 30-1, 30-2 which confronts the focusing magnet26-1, 26-2 is maximized at the center of the magnet, and thus themagnetic substance 30-1, 30-2 is magnetically balanced and stable at theposition corresponding to the center of the focusing magnet. Payingattention to the flow of the magnetic flux in the height direction, themagnet substance 30-1, 30-2 is magnetically balanced and stable in theneighborhood of the boundary of the N and S poles of the focusing magnet26-1, 26-2 so as to form a magnetic loop of the N pole of the focusingmagnet 26-1, 26-2→ the magnetic substance 30-1,30-2→S pole→N pole.Accordingly, the objective lens 21 can be stably positioned by themagnetic balance in the rotational direction (tracking direction) andthe height direction (focusing direction) with respect to the slidingshaft 25.

SUMMARY OF THE INVENTION

[0012] Recently, there has been adopted a method of increasing thenumber of rotation of an optical disc so as to increase the datatransfer rate from an optical disc apparatus, and it has become more andmore necessary to enhance the follow-up property (responsibility) of theobjective lens of the objective lens driving device to the planevibration and the track eccentricity of the recording surface of theoptical disc. Particularly, the acceleration of distortion due to theeccentricity of the track is rapidly increased because it isproportional to the square of the number of rotation, and thus thefollow-up performance of the objective lens in the tracking direction isrequired to be enhanced. A method for intensifying the electromagneticforce of the coil in a gap of a magnetic circuit may be considered toenhance the follow-up performance of the objective lens. In order tointensify the electromagnetic force of the coil, the effective portionof the coil in the gap of the magnetic circuit may be lengthened.However, in the objective lens driving device of the first prior artshown in FIGS. 9 to 11, the lowermost portion of the tracking coil 4 inthe sliding axis direction is set to be equal to the lowermost portionof the focusing coil 5 in height. Therefore, in order to prevent theoptical beam 8 from being intercepted by the coil when the effectiveportion of the coil is lengthened, it is necessary that the optical beam8 is disposed so as to pass over the lower side of the focusing coil 5and the tracking coil 4 in the sliding shaft direction, or incident fromthe optical axis direction of the objective lens 7. Consequently, theobjective lens driving device is bulky, and thus the optical discapparatus must be designed in a large size. Further, the optical discapparatus is also required to be compact in addition to the requirementof the enhancement of the follow-up performance of the objective lens ofthe objective lens driving device to the plane vibration and the trackeccentricity of the recording surface of the optical disc. Therefore,the objective lens driving device is required to be thin and compact.According to the first objective lens of the first prior art, if thefollow-up performance of the objective lens is enhanced, the objectivelens driving apparatus would be bulky (thick). That is, it is impossibleto satisfy both the requirement for the enhancement of the follow-upperformance of the objective lens and the requirement for the thin(low-profile) and compact design of the objective lens driving device.

[0013] According to the present invention, an objective lens drivingdevice comprises a movable unit having an objective lens for focusing anoptical beam onto an optical disc, a focusing coil for driving theobjective lens in an optical axis direction thereof, a tracking coil fordriving the objective lens in a radial direction of the optical disc anda lens holder for holding the objective lens, the focusing coil and thetracking coil, a sliding shaft which is provided substantially inparallel to the optical axis direction of the objective lens and adaptedto guide the movable unit, a magnetic circuit for generating magneticflux which is applied to the focusing coil and the tracking coil, and amirror for converting the optical beam in the optical axis direction ofthe objective lens, wherein the tracking coil is disposed outside theobjective lens in the radial direction with respect to the slidingshaft, and the whole or part of the optical beam incident to the mirrorpasses through a space surrounded by the tracking coil and the focusingcoil which are adjacent to each other, thereby achieving both theenhancement of the follow-up performance to the optical disc and thethin and compact design of the objective lens driving device.

[0014] Recently, the standards of optical discs which have highrecording density and are different in substrate thickness have beenannounced, and they have just started to require optical discapparatuses which can reproduce not only CDs and CD-ROMs, but also theseoptical discs having high recording density. Therefore, in order tomatch both the two types of optical discs which have different recordingdensity or substrate thickness, there may be considered such a type ofobjective lens driving device that two kinds of objective lenscorresponding to the respective types of optical discs are provided andthe objective lens being used is switched in accordance with the type ofoptical disc. In this case, if the magnetic circuit and the positioningmechanism of the second prior art described above is used, the coil, themagnet and the magnetic substance must be disposed on the samecircumference to form a magnetic circuit, and thus the objective lensdriving device must be designed in a large shape. Further, it isdifficult to use the magnetic circuit commonly, and the number of partscan be expected to increase because these parts must be usedexclusively.

[0015] The present invention has been implemented in order to solve theabove problems, and has an object to provide an objective lens drivingdevice having an objective lens switching mechanism in which thepositioning of the objective lens in a focusing direction and a trackingdirection can be performed with high reproducibility and high precisioneven after the objective lens is switched to another, and which can alsoreduce the number of parts to lower the cost.

[0016] Therefore, the objective lens driving device according to thepresent invention is adapted to the two types of optical discs which aredifferent in recording density, and it includes two types of objectivelens which are respectively adapted to focus an optical beam onto anoptical disc and are matched with the two types of optical discs, a lensholder for holding a focusing coil and tracking coils for driving theobjective lens in the focusing and tracking directions and magneticsubstances, and a sliding shaft for sliding and rotating the lens holderin the focusing direction and the tracking direction. Further, theobjective lens driving device is provided with a magnet and a yoke whichare fixed to confront the coils and keep a magnetic gap to thereby applymagnetic flux to the coils, and the magnetic substances are radiallydisposed out of the magnetic gap on a plane perpendicular to the planedefining the magnetic gap, whereby a magnetic spring is constituted byleakage flux of the magnetic circuit.

[0017] In general, the objective lens driving device is provided withcurrent supply means such as FPC or the like for supplying current tothe movable portion comprising the lens holder, the coils, etc. In theobjective lens driving device having the objective lens switchingmechanism, the current supply means is greatly bent in an interlockingmanner with the objective lens switching operation. The restoring forceof the current supply means from the bent state to the initial stateacts on the movable portion in proportion to the switching amount of theobjective lens. Therefore, as the objective lens switching angle is setto a larger value in order to further miniaturize the objective lensdriving device, the effect of the restoring force by the current supplymeans becomes less negligible as comparison with the restoring force ofthe magnetic spring to the neutral position, and the precision of thepositioning of the objective lens is lowered.

[0018] The present invention has been implemented to solve the aboveproblem, and has an object to suppress the lowering of the positioningprecision of the objective lens using the restoring force of the currentsupply means in the objective lens driving device which is provided withplural objective lens and an objective lens switching mechanism.

[0019] In order to solve the above problem, according to the presentinvention, an objective lens driving device includes plural objectivelenses for focusing an optical beam, a lens holder for holding theobjective lenses, a focusing coil which is provided in the lens holderto drive the objective lenses in a focusing direction, tracking coilswhich are provided in the lens holder to drive the objective lenses in atracking direction, a sliding shaft for guiding the lens holder slidablyand rotatably, and a yoke and plural magnets for forming a magneticcircuit having a magnetic gap in which the focusing coil and thetracking coils are inserted, wherein magnetic substances are disposed orformed on the lens holder in a radial direction with the sliding shaftlocated at the rotational center, and the angles at which the respectiveobjective lenses are located around the shaft center of the slidingshaft (i.e., the location angles of the objective lens with the shaftcenter of the sliding shaft) is set to be varied with respect to thelocation angles of the magnetic substances around the shaft center ofthe sliding shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is an explanatory configuration diagram showing a firstembodiment of an objective lens driving device according to the presentinvention;

[0021]FIG. 2 is a cross-sectional view of the configuration of theobjective lens driving device which is taken along a line A-A line ofFIG. 1;

[0022]FIG. 3 is an explanatory configuration diagram showing theobjective lens driving device which is viewed from the direction of anarrow B of FIG. 1;

[0023]FIG. 4 is an explanatory configuration diagram showing a secondembodiment of the objective lens driving device according to the presentinvention;

[0024]FIG. 5 is an explanatory configuration diagram showing theobjective lens driving device which is viewed from the direction of anarrow D of FIG. 4;

[0025]FIG. 6 is a diagram showing the state of the objective lensdriving device of the second embodiment;

[0026]FIG. 7 is an explanatory configuration diagram showing theobjective lens driving device which is viewed from the direction of anarrow F of FIG. 6;

[0027]FIG. 8 is an explanatory configuration diagram showing a thirdembodiment of the objective lens driving device according to the presentinvention;

[0028]FIG. 9 is an explanatory configuration diagram showing aconventional objective lens driving device;

[0029]FIG. 10 is a cross-sectional view which is taken along a line GGof FIG. 9;

[0030]FIG. 11 is an explanatory configuration diagram of theconventional objective lens driving device which is viewed from thedirection of an arrow H of FIG. 9;

[0031]FIG. 12 is an explanatory configuration diagram showing anotherconventional objective lens driving device;

[0032]FIG. 13 is a cross-sectional view which is taken along a line K-Kof FIG. 12;

[0033]FIG. 14 is an explanatory configuration diagram showing fourth andseventh embodiments of the objective lens driving device according tothe present invention, and shows the state where a first objective lens7-1 corresponding to one of two types of optical discs is selected;

[0034]FIG. 15 is a cross-sectional view which is taken along a line I-Iline of FIG. 14;

[0035]FIG. 16 is a cross-sectional view which is taken along a line J-Jof FIG. 14;

[0036]FIG. 17 is a diagram showing the fourth and seventh embodiments ofthe objective lens driving device of the present invention, and showsthe state where a second objective lens 7-2 corresponding to the otherof the two types of optical discs is selected;

[0037]FIG. 18 is a cross-sectional view which is taken along a line I-Iof FIG. 14, and shows the state where an optical disc mounted in acartridge is used;

[0038]FIG. 19 is an exploded perspective view showing the fourth andseventh embodiments of the objective lens driving device according tothe present invention;

[0039]FIG. 20 is a diagram showing the state of the fourth and seventhembodiments of the objective lens driving device according to thepresent invention, and shows the state where the first objective lens 71corresponding to one of the two types of optical discs is selected;

[0040]FIG. 21 is a diagram showing the fourth and seventh embodiments ofthe objective lens driving device according to the present invention,and shows the state where the second objective lens 7-2 corresponding tothe other of the two types of optical discs is selected;

[0041]FIG. 22 is an explanatory configuration diagram showing an eighthembodiment of the objective lens driving device according to the presentinvention;

[0042]FIG. 23 is an explanatory configuration diagram showing fifth andninth embodiments of the objective lens driving device according to thepresent invention, and shows the state where the first objective lens7-1 corresponding to one of the two types of optical discs is selected;

[0043]FIG. 24 is an explanatory configuration diagram showing sixth andtenth embodiments of the objective lens driving device according to thepresent invention, and shows the state where the first objective lens7-1 corresponding to one of the two types of optical discs is selected;

[0044]FIG. 25 is an explanatory diagram showing necessary torque in thetracking direction in the objective lens driving device of FIG. 14;

[0045]FIG. 26 is an explanatory diagram showing the force in thefocusing direction in the objective lens driving device of FIG. 14;

[0046]FIG. 27A is a diagram showing the shape of a notched end portionof an inner yoke;

[0047]FIG. 27B is a diagram showing the shape of a notched end portionof an inner yoke;

[0048]FIG. 27C is a diagram showing the shape of a notched end portionof an inner yoke;

[0049]FIG. 28 is an explanatory diagram showing the effect of the heightvariation in the focusing direction on the displacement in the trackingdirection in the objective lens driving device of FIG. 14; and

[0050]FIG. 29 is a diagram showing an improvement in characteristics ofFIG. 28.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] Preferred embodiments according to the present invention will bedescribed hereinafter with reference to the accompanying drawings.

[0052] A first embodiment of an objective lens driving device accordingto the present invention will be described first.

[0053]FIG. 1 is a configuration diagram showing a first embodiment of anobjective lens device according to the present invention. FIG. 2 shows across-section of FIG. 1 along a line A-A. FIG. 3 is a drawing showingthe objective lens driving device which is viewed from the direction ofan arrow B of FIG. 1.

[0054] In FIG. 1, one objective lens 7 is provided in a lens holder 3,and the lens holder 3 is supported and guided by a sliding shaft 6 whichis slidably and rotatably disposed substantially in parallel to theoptical axis direction of the objective lens 7. A focusing coil 5 iswound around the outer peripheral surface of the lens holder 3, andtracking coils 4 are provided substantially coaxially at angularintervals of substantially 90° on the outer peripheral portion of thelens holder 3. At this time, the lowermost portions of the trackingcoils 4 are disposed so as to be nearer to a mirror 10 than thelowermost portion of the focusing coil 5 as shown in FIGS. 2 and 3.Further, a magnetic circuit comprising magnets 2, outer yokes 1 andinner yokes 9 is provided to generate magnetic flux so that the magneticflux traverses the tracking coils 4 and the focusing coil 5.

[0055] In the above constitution, at least one part of the optical beam8 passes through the space which is surrounded by the tracking coils 4and the focusing coil 5 which are partially adjacent to each other (inanother aspect, the space within the dimension of the tracking coils inthe optical direction of the objective lens), and then becomes incidentto the mirror 10 by which the optical path is deflected in the opticalaxis direction of the objective lens 7.

[0056] In the focusing control operation, by causing the current to flowinto the focusing coil 5 in the forward or reverse direction, theelectromagnetic force acts in the direction of the sliding shaft 6 inthe gap of the magnetic circuit to drive the objective lens 7 in theoptical axis direction in accordance with the plane vibration of therecording surface of the optical disc, whereby the spot of the opticalbeam 8 can follow the recording surface of the optical disc. Further, inthe tracking control operation, by causing the current to flow into eachtracking coil 4 in the forward or reverse direction, the electromagneticforce acts as a rotational couple of force around the sliding shaft 6 inthe gap of the magnetic circuit , whereby the spot of the optical beam 8can follow the eccentricity of the track of the optical disc. As thelength of the effective portion of the tracking coil 4 in the gap of themagnetic circuit, that is, the height of the tracking coil 4 in thedirection of the sliding shaft is increased, the driving force of thetracking coil 4 is stronger. This effect enables the objective lens 7 tofollow the eccentricity of the track of the optical disc of theobjective lens driving device.

[0057] As described above, in the objective lens driving deviceaccording to this embodiment, the lowermost portions of the trackingcoils 4 are set to be lower than the lowermost portion of the focusingcoil 5 in the direction of the sliding shaft, and a part of the opticalbeam 8 is passed through the space surrounding the neighboring trackingcoil 4 and the focusing coil 5 from the side surface of the objectivelens driving device to enable the objective lens driving device to bedesigned in a low-profile (thin) design. Further, the effective portionof the tracking coil 4 which intersects the magnetic flux can be keptlong, so that the driving force can be magnified, and the follow-upperformance during the high-speed rotation of the optical disc can beenhanced.

[0058] Next, a second embodiment of the objective lens driving deviceaccording to the present invention will be described.

[0059]FIG. 4 is an explanatory configuration diagram showing theconstruction of a second embodiment, FIG. 5 is a diagram which is viewedfrom the direction of an arrow D of FIG. 4, FIG. 6 is a state diagram ofthis embodiment, and FIG. 7 is a diagram which is viewed from thedirection of an arrow F of FIG. 6.

[0060] As shown in FIG. 4, this embodiment is different from the firstembodiment in that a first objective lens 7-1 and a second objectivelens 7-2 are provided in the lens holder 3 substantially at angularintervals of 90°. The other construction is the same as the firstembodiment, and thus the description thereof is omitted. In FIGS. 4 to7, the first objective lens 7-1 and the second objective lens 7-2 areprovided to conform to two different types of optical discs (forexample, DVD and CD), respectively. By switching the objective lens inaccordance with the type of the optical disc, different types of opticaldiscs can be supported by only one objective lens driving device. Theswitching operation of the objective lens is carried out by causingpulsed current to flow into the tracking coils 4. FIG. 4 shows the statewhere the first objective lens 7-1 which conforms to one (for example,DVD) of the two types of optical discs is selected, and FIG. 6 shows thestate where the second objective lens 7-2 which conforms to the otheroptical disc (for example, CD) of the two types of optical discs isselected As shown in FIGS. 5 and 7, as in the case of the firstembodiment, in a condition where any of the objective lenses areselected by switching of the first objective lens and the secondobjective lens in accordance with the type of the optical disc, a partof the optical beam 8 can be passed through the space surrounded by theneighboring tracking coils 4 and focusing coil 5 from the side surfaceof the objective lens driving device, and thus the objective lensdriving device can be designed in a low-profile structure. Further, theeffective portion of the tracking coil which intersects the magneticflux can be kept long, so that the driving force can be magnified andthe follow-up performance during the high-speed rotation of the opticaldisc can be enhanced as in the case of the first embodiment.

[0061] Next, a third embodiment of the objective lens driving deviceaccording to the present invention will be described.

[0062]FIG. 8 is a diagram showing the third embodiment according to thepresent invention. The main construction of the third embodiment is thesame as the first embodiment, but, the third embodiment is different inthe first embodiment in that the overall optical beam 8 is passed fromthe side surface of the objective lens driving device through the spacewhich is surrounded by the tracking coils 4 and the focusing coil 5which are adjacent to each other. The overall optical beam 8 can bepassed from the objective lens driving device through the spacesurrounded by the tracking coils 4 and the focusing coil 5 which areadjacent to one another as in the case of the first embodiment, and thusthe objective lens driving device can be designed in a low-profile(thin) structure. Further, the effective portion of the tracking coils 4which intersect the magnetic flux can be kept long, so that the drivingforce can be magnified and the follow-up performed during the high-speedrotation of the optical disc can be enhanced. The same effect as thefirst embodiment can be obtained in the third embodiment. Further, as inthe case of the second embodiment, both the first and second objectivelenses can be provided on the lens holder in the third embodiment, sothat the same effect as the second embodiment can be obtained.

[0063] As described above, according to the present invention, the wholeor a part of the optical beam 8 can be passed through the spacesurrounded by the tracking coils 4 and the focusing coil 5 which arefixed to the lens holder 3 and adjacent to each other, whereby theenhancement of the follow-up of the objective lens to the optical discand the thin and compact design of the objective lens driving device canbe performed at the same time.

[0064] Next, a fourth embodiment of the objective lens driving deviceaccording to the present invention will be described.

[0065]FIG. 14 is an explanatory configuration diagram showing theobjective lens driving device of the fourth embodiment, and shows thestate where the first objective lens 7-1 which conforms to one of thetwo types of optical discs is selected. FIG. 15 is a cross-sectionalview which is taken along a line I-I of FIG. 14, FIG. 16 is across-sectional view which is taken along a line J-J of FIG. 14, andFIG. 17 shows the state where the second objective lens 7-2 whichconforms to the other optical disc of the two types of optical discs isselected.

[0066] In FIGS. 14 to 17, reference numeral 1 represents an outer yoke,reference numeral 2 represents a magnet, reference numeral 3 representsa lens holder, reference numeral 4 represents a tracking coil, referencenumeral 5 represents a focusing coil, reference numeral 6 represents asliding shaft, reference numeral 7-1 represents a first objective lens,reference numeral 7-2 represents a second objective lens, referencenumeral 9 represents an inner yoke, and reference numerals 12-1, 12-2,12-3, 12-4 represent magnetic substances.

[0067] The lens holder 3 in which the first objective lens 7-1 and thesecond objective lens 7-2 are held substantially at angular intervals of90° is freely rotatably and slidably secured to the sliding shaft 6. Thelens holder 3 holds one focusing coil 5 for driving the objective lensin the height direction to the sliding shaft 6, and four tracking coils4 which are disposed substantially at four angular intervals of 90° andadapted to drive the objective lens in the rotational direction, andfurther holds four magnetic substances 12-1 to 12-4 each formed of arectangular plate so that these magnetic substances are arranged in theradial direction to the shaft center of the sliding shaft 6 andsubstantially at angular intervals of 90°, thereby constituting amovable unit.

[0068] The inner yokes 9 are fixed on the inner peripheral sides of thefocusing coil 5 and the tracking coils 4, and the magnets 2 which aremagnetized to have unipolarity and the yokes 1 are disposedsubstantially at an angular interval of 90° are fixed at the outerperipheral sides of the focusing coil 5 and the tracking coils 4,thereby constructing the magnetic circuit which forms a magnetic loop ofthe magnet 2→the tracking coil 4→the focusing coil 5→the inner yoke9→the outer yoke 1→the magnet 2, to apply magnetic flux to both thecoils. The inner yoke 9 is provided with a notch portion to guide theoptical beam 8 to the first objective lens 7-1 or the second objectivelens 7-2, and the optical beam 8 is passed through the notch portion ofthe inner yoke 9. Further, in order to make the magnetic circuitsymmetrical, a notch portion is also provided on the inner yoke portionat the opposite side. Here, the magnetic substances 12-1 to 12-4 areheld at the upper side of the inner yokes 9 and on the lower surface ofthe lens holder 3.

[0069] In the magnetic circuit thus constructed, the magnetic fluxdensity distribution in the rotational direction (tracking direction) atthe upper side of the inner yoke 9 which confronts the magnet 2 becomesmaximum at the center portion of the magnet 2. Accordingly, each of themagnetic substances 12-1 to 12-4 which are disposed substantiallyradially are attracted toward the center of the corresponding magnet 2.Therefore, the magnetic substances are magnetically balanced under thepositional relationship as shown in FIG. 14, so that the first objectivelens 7-1 and the second objective lens 7-2 can be positioned stably inthe rotational direction (tracking direction). Further, the positioningprecision can be more enhanced by forming each of the magneticsubstances 12-1 to 12-4 integrally with the lens holder 3.

[0070]FIG. 25 shows the relationship between the rotational angle θ ofthe lens holder 3 and the torque T which is needed to rotate the lensholder 3 of the fourth embodiment. The state of FIG. 14 corresponds tosuch a condition that the rotational angle on the abscissa is equal to0°, and the state of FIG. 17 corresponds to such a condition that theobjective lens is switched to the second objective lens 7-2 and therotational angle is equal to 90°. An area in which the rotational angleis about 45° corresponds to an unsteady area where the sign of thetorque is inverted. If the rotational angle exceeds this unsteady area,the next magnetic substance piece is attracted and the stable magneticbalance is established again at the position where the rotational angleis equal to 90°.

[0071] Accordingly, by supplying the current to the tracking coils 4 ina pulse form to rotate the lens holder 3 by 45° or more, according tothe relationship in force shown in FIG. 25, the objective lens isswitched from the state of FIG. 14 where the first objective lens 7-1 isselected, to the state of FIG. 17 where the second objective lens 7-2 isselected, whereby the switching operation can be performed with highreliability. In addition, the positioning can be performed with highprecision. Here, by changing the thickness of the magnetic substances12-1 to 12-4 arranged in a cross, the spring constant of the magneticspring which is expressed by the gradient of a linear portion of FIG. 25can be changed.

[0072]FIG. 26 shows the relationship between a force required to slidethe lens holder 3 in the focusing direction and the height of the lensholder 3. The abscissa represents the height H of the lens holder 3, andthe ordinate represents the needed force F.

[0073] When the magnetic substances 12-1 to 12-4 and the magnets 2 arelocated in the same positional relationship shown in FIG. 14, themagnetic substances 12-1 to 12-4 are magnetically balanced in height h1.At this time, it has been experimentally proved that the relationshipbetween the force F and the height (displacement) H varies linearly asin the case of the rotational direction, and the spring constant of themagnetic spring which is expressed by the gradient of the linear portionis dependent on the projection area of each piece of magnetic substance12-1 to 12-4 in the sliding direction (focusing direction) of the lensholder 3. Accordingly, the shape of the magnetic substances 12-1 to 12-4can be determined from the conditions of a servo system, etc.

[0074] The shapes of the inner yokes 9 which confront the magneticsubstances 12-1, 12-2, 12-3, 12-4 are shown in FIG. 27A and FIG. 27B inthe magnetic circuit of the fourth embodiment. In the magnetic circuit,if the corner portion 31 of the notch portion of the inner yoke 9 whichconfronts the magnetic substance 12-1, 12-2, 12-3, 12-4 issquare-cornered as shown in FIG. 27A, the magnetic flux is liable to beconcentrated on the ridgeline of the corner portion 31 to increase themagnetic flux density there. At this time, when the lens holder 3 isslid in the focusing direction to approach the magnetic substances 12-1,122, 12-3, 12-4 to the corner portion 31 of the notch portion of theinner yoke 9, the magnetic substances 12-1, 12-3 which are nearest tothe corner portion 31 are attracted to the corner portion of the inneryokes 9 having high magnetic flux density as indicated by an arrow S,although its attraction level is small. FIG. 28 is a diagram showingthis phenomenon. In FIG. 28, the ordinate represents the height of thelens holder 3 in the focusing direction, and the abscissa represents thedisplacement in the tracking direction of the lens holder 3. It is foundthat as the height of the lens holder 3 in the focusing direction islowered, the lens holder 3 is displaced in the tracking direction. Thedirection of the displacement corresponds to the direction in which thecenter lines of the magnetic substances 12-1, 12-3 are attracted to thecorner portion 31 of the notch portion of the inner yoke 9. Occurrenceof such a phenomenon intensifies external perturbations on the servo inthe tracking direction.

[0075] On the other hand, when the corner portion of the notch portionof the inner yoke 9 which confronts the magnetic substance 12-1, 12-2,12-3, 12-4 is designed in an R (round) shape (R0 to R2) as shown in FIG.27B, it has been experimentally found that the magnetic fluxdistribution around the rounded corner portion has a lower magnetic fluxdensity as compared with that of the squared corner portion.Accordingly, even when the magnetic substance 12-1, 12-2, 12-3, 12-4approaches to the R-shaped portion 32 of the notched portion of theinner yoke 9, the magnetic substances 12-1, 12-3 are not attracted tothe R-shaped portion 32. FIG. 29 is a diagram showing this phenomenon.In FIG. 29, the ordinate represents the height of the lens holder 3 inthe focusing direction, and the abscissa represents the displacement ofthe lens holder 3 in the tracking direction. By designing the cornerportion of the notch portion of the inner yoke 9 in an R shape (R0 toR2) as shown in FIG. 27B, the lens holder 3 is only slightly displacedin the tracking direction even when it is vertically moved in thefocusing direction. Therefore, according to this constitution, thehigh-precision tracking servo can be achieved, and the driving devicehaving high reliability can be achieved.

[0076] Further, the same effect as shown in FIG. 29 can be obtained bysufficiently lengthening the inner yokes 9 confronting the magneticsubstances 12-1, 12-2, 12-3, 12-4 to increase the overlap amount, thatis, by designing the magnetic circuit so that the magnetic substances12-1, 12-2, 12-3, 12-4 suffers no effect of the magnetic flux around thenotch portion of the inner yokes 9 as show in FIG. 27C.

[0077]FIG. 23 is an explanatory configuration diagram showing a fifthembodiment of the objective lens driving device according to the presentinvention, and shows the state where the first objective lens 7-1 whichconforms to one of the two types of optical discs is selected. In FIG.23, the same elements as the fourth embodiment are represented by thesame reference numerals and the description thereof is omitted to avoidrepetition.

[0078] The difference of the fifth embodiment from the fourth embodimentresides in that the fourth embodiment uses four magnetic substances 12-1to 12-4, but, the fifth embodiment uses one substantially cross-shapedplanar magnetic substance 12 in place of the four magnetic substances.Although the four magnetic substances 12-1 to 12-4 of the fourthembodiment are replaced by only one substantially cross-shaped planarmagnetic substance 12 in this embodiment, the same effect as the fourthembodiment can be achieved and the number of pieces of magneticsubstance can be reduced.

[0079]FIG. 24 is a diagram showing a sixth embodiment of the objectivelens driving device according to the present invention, and shows thestate where the first objective lens 7-1 which conforms to one of thetwo types of optical discs is selected. In FIG. 24, the same componentsas the fifth embodiment are represented by the same reference numerals,and the description thereof is omitted to avoid repetition.

[0080] The construction of this embodiment is substantially identical tothat of the fifth embodiment, but, this embodiment is different from thefifth embodiment in that a notch to keep a passage for an optical beamincident to the objective lens is provided for each of four pieces ofthe substantially cross-shaped magnetic substance 12 of this embodiment.With the construction of the sixth embodiment as described above, notonly can the same effect as the fifth embodiment be achieved, but alsoan objective lens having a large aperture diameter which is suitablyused for an optical disc having high recording density can be mountedwithout obstructing the compact design of the objective lens drivingdevice.

[0081] If a notch for keeping a passage for an optical beam incident tothe objective lens is provided for each of the four magnetic substances12-1 to 12-4 in the fourth embodiment, an object lens having a largeaperture diameter which is suitably used for an optical disc having highrecording density can be mounted without obstructing the compact designof the objective lens driving device as in the case of the sixthembodiment.

[0082] As described above, according to the fourth to sixth embodiments,in the objective lens driving device having the switching mechanism forthe two types of objective lens which conform to the two types ofoptical discs respectively, the magnetic substances are disposed withina plane perpendicular to the plane constituting the magnetic gap in aradial direction outside of the magnetic gap to form the magnetic springby using the leakage magnetic flux of the magnetic circuit, whereby theobjective lens in the focusing direction and the tracking direction canbe reproduced with high reproducibility and high precision even afterthe switching operation of the objective lens is performed. Further, thenumber of parts can be reduced to lower the cost.

[0083] Next, a seventh embodiment according to the present inventionwill be described.

[0084]FIG. 14 is a diagram showing a seventh embodiment, and shows thestate where the first objective lens 7-1 is selected. FIG. 15 is across-sectional view which is taken along a line I-I of FIG. 14, andFIG. 16 is a cross-sectional view which is taken along a line J-J ofFIG. 14. FIG. 17 is an explanatory configuration diagram showing thestate where the second objective lens 7-2 which is different from thatof FIG. 14 is selected, and FIG. 18 is a cross-sectional view takenalong a line I-I of FIG. 14 and shows the state where a cartridgecontains an optical disc. FIG. 19 is an exploded perspective viewshowing this embodiment. FIG. 20 is a status diagram of this embodiment,and shows the state where the first objective lens 7-1 is selected. FIG.21 is a diagram explaining a state where the second objective lens 7-2is selected.

[0085] In FIGS. 14 to 19, the first objective lens 7-1, the secondobjective lens 7-2, the magnetic substance 12, the focusing coil 5 andthe tracking coil 4 are mounted on the lens holder 3 which is freelyslidable and rotatable around the sliding shaft 6, thereby forming themovable unit. The maximum length 15 of the movable unit is set to besmaller than the length of the corresponding cartridge open portion 17as shown in FIG. 18, and the first objective lens 7-1 and the secondobjective lens 7-2 are disposed at a lens arrangement angle 20 aroundthe shaft center of the sliding shaft 7-2. In this case, the objectivelens arrangement angle 20 is set to be smaller than substantially 90°.The four magnetic substances 12-1 to 12-4 are radially disposedsubstantially at an angular interval of 90° around the shaft center ofthe sliding shaft 6. The outer yokes 1, the inner yokes 9 and themagnets 2 constitute a magnetic circuit. The focusing coil 5 and thefour magnets 2 confronting the four tracking coils 4 are magnetized inthe radial direction, and they are arranged substantially at the samearrangement angle 13 as the magnetic substances 12. The magnetic fluxdensity distribution in the peripheral direction around the magneticsubstances 12 is maximum at the center portion of the confrontingmagnets 2, and thus the magnetic substances 12 are magnetically balancedand stabilized around the position confronting the center of the magnets2. Further, the magnetic substances 12 are magnetically balanced andstabilized at the upper sides of the inner yokes 9 so as to form amagnetic loop in which the magnetic flux in the height direction flowsin the direction of the magnet 2→the magnetic substance 12→the inneryoke 9→the outer yoke 1→the magnet 2, whereby the overall movable unit,the first objective lens 7-1 and the second objective lens 7-2 arepositioned stably and with high precision by the magnetic spring whichis formed by the magnetic substances 12 and the magnetic circuit. FPC 11links the outer yokes 1 and the movable unit so that the sectional shapeis substantially U-shaped. By supplying a predetermined amount ofcurrent to the focusing coil 5 and the tracking coils 4 through the FPC11, each coil in the magnetic circuit can obtain a driving force whichis called “Fleming's force”, and the first objective lens 7-1 and thesecond objective lens 7-2 which are integral with the lens holder 3 aremovable in the height direction and in the peripheral direction. As aresult, the light flux incident from the mirror 10 to the firstobjective lens 7-1 can be converged onto the optical disc (not shown).In the first objective lens 7-1, when an optical disc having no functionof converging the light is mounted, the second objective lens 7-2 isrotated to be disposed just above the mirror 10, as shown in FIG. 17, byinstantaneously supplying rectangular-wave current to the tracking coils4, and the light can be converged onto the optical disc by using thesecond objective lens 7-2.

[0086] Next, the behavior in the lens switching operation will bedescribed with reference to FIGS. 20 and 21.

[0087] In FIG. 20, the first objective lens 7-1 is selected and locatedjust above the mirror 10. When the second objective lens 7-2 is selectedto support another type of optical disc in the above state, the lensholder 3 in which the first objective lens 7-1 and the second objectivelens 7-2 are mounted is rotated about the shaft center of the slidingshaft 6 by driving means (not shown). Since the movable unit isphysically linked to the outside by the FPC 11, the distortion amount ofthe FPC 11 is increased as the switching angle of the objective lens,that is, the objective lens arrangement angle 20 on the lens holder 3,is increased. Since the distortion of the FPC 11 acts as a resistanceforce to the movable unit, the rotational angle of the movable unit issmaller than substantially 90° by only an angle which is represented bya displacement angle 19. In the present invention, the objective lensarrangement angle 20 is set to be smaller than the magnetic circuitarrangement angle 13 by he angle corresponding to the displacement angle19, so that the second objective lens 7-2 can be located just above themirror 10 as shown in FIG. 21.

[0088] As described above, according to the seventh embodiment, theobjective lens arrangement angle 20 is set to be smaller than themagnetic circuit arrangement angle 13 by only the displacement angle 19due to the resistance force of the FPC 11, so that a plurality ofobjective lenses can be stably positioned just above the mirror 10 withhigh precision. Further, the positioning can be performed with higherprecision by forming the magnetic substances integrally with the lensholder.

[0089] Next, an eighth embodiment of the objective lens driving deviceaccording to the present invention will be described.

[0090]FIG. 22 is an explanatory configuration diagram showing theconstitution of the eighth embodiment. The difference of this embodimentfrom the seventh embodiment resides in that two magnetic substances12-1, 12-2 are formed. The other construction is identical to that ofthe seventh embodiment, and the description thereof is omitted. The twomagnetic substances 12-1, 12-2 are disposed symmetrically with respectto the shaft center of the sliding shaft 6. In this case, the magneticcircuit arrangement angle 13 indicates an angle which is formed aroundthe shaft center of the sliding shaft 6 by two magnets 2 which confronta specific magnetic substance 12 when one of the first objective lens7-1 and the second objective lens 7-2 is selected. As in the case of theseventh embodiment, by setting the objective lens arrangement angle 20to be smaller than the magnetic circuit arrangement angle 13, the sameeffect as in the seventh embodiment described above can be obtained. Inaddition, the number of parts can be reduced and the efficiency of thefabrication process can be enhanced.

[0091] Next, a ninth embodiment of the objective lens driving deviceaccording to the present invention will be described.

[0092]FIG. 23 is a diagram showing the ninth embodiment. The differenceof this embodiment from the seventh and eighth embodiments resides inthat a substantially cross-shaped magnetic substance 12 is provided inplace of plural magnetic substances 12. The other construction isidentical to that of the seventh and eighth embodiment, and the detaileddescription thereof is omitted. As in the case of the seventhembodiment, the objective lens arrangement angle 20 is set to be smallerthan the magnetic circuit arrangement angle 13 in this embodiment,whereby the same effect as the first embodiment can be achieved and thenumber of parts can be reduced to enhance the efficiency of thefabrication process.

[0093] Next, a tenth embodiment of the objective lens driving deviceaccording to the present invention will be described.

[0094]FIG. 24 is an explanatory configuration diagram showing theconstruction of the tenth embodiment. The difference of the tenthembodiment from the ninth embodiment resides in that a notch portion isprovided on a part of the magnetic substance 12 of FIG. 23 so as to besymmetrical with respect to the shaft center of the sliding shaft 6, sothat an objective lens 21 having a larger diameter can be mounted.According to this embodiment, as in the case of the ninth embodiment,the objective lens arrangement angle 20 is set to be smaller than themagnetic circuit arrangement angle 13, so that the same effect as theseventh embodiment can be achieved and the number of parts can bereduced to enhance the efficiency of the fabrication process.

[0095] In the seventh to tenth embodiments, the current supply to thefocusing coil and the tracking coils is performed by a method using FPC.However, the present invention is not limited to this method. Forexample, the current supply may be performed by lead wires which areindependently extended from the respective terminals of the focusingcoil 5 and the tracking coils 4. In the above description, the objectivelens arrangement angle 20 is assumed to be smaller than the magneticcircuit arrangement angle 13. However, when the direction of theresistance force of the current supply means is different from that ofthe foregoing description, it is needless to say that the objective lensarrangement angle 20 is set to be larger than the magnetic circuitarrangement angle 13. Further, the number of the objective lens is notlimited to two, and it may be equal to three or more. In the foregoingdescription, the magnetic circuit arrangement angle 13 is assumed to beset to substantially 90°. However, the magnetic circuit arrangementangle 13 is not limited to this value.

[0096] As described above, according to the present invention, in theobjective lens driving device having the mechanism for switching andselecting one of two or more objective lens in order to support pluraltypes of optical discs, the two objective lens on the lens holder arearranged while the objective lens arrangement angle thereof is displacedwith respect to the objective lens switching angle which is determinedby a combination of the magnetic substances and the magnetic circuit,whereby the objective lens position which is displaced due to theresistance force of the current supply means such as FPC or the like canbe properly corrected, and the objective lens in the lens switchingoperation can be positioned with high precision.

What is claimed is:
 1. An objective lens driving device comprising: amovable unit having an objective lens for focusing an optical beam ontoan optical disc; a focusing coil for driving said objective lens in anoptical axis direction thereof; tracking coils for driving saidobjective lens in a radial direction of the optical disc and a lensholder for holding said objective lens, said focusing coil, and saidtracking coils; a sliding shaft which is provided substantially inparallel to the optical axis direction of said objective lens andadapted to support and guide said movable unit; a magnetic circuit forgenerating magnetic flux which is applied to said focusing coil and saidtracking coils; and a mirror for converting the optical beam in theoptical axis direction of said objective lens; wherein each of saidtracking coils is disposed outside said objective lens in the radialdirection with respect to said sliding shaft, and the whole or part ofthe optical beam incident to said mirror passes through a spacesurrounded by said tracking coils and said focusing coil which areadjacent to each other.
 2. The objective lens driving device as claimedin claim 1, wherein when Lt represents the dimension of each of saidtracking coils in the optical axis direction of said objective lens andLf represents the dimension of said focusing coil in the opticaldirection of said objective lens, said objective lens driving device isdesigned so as to satisfy Lt>Lf.
 3. The objective lens driving device asclaimed in claim 1, wherein the lowermost portion of each of saidtracking coils in the optical axis direction of said objective lens isnearer to said mirror than the lowermost portion of said focusing coilin the optical direction of said objective lens.
 4. The objective lensdriving device as claimed in claim 1, wherein said holder is providedwith a plurality of said objective lenses, and said objective lenses aremutually switched in accordance with the type of the optical disc. 5.The objective lens driving device as claimed in claim 1, wherein saidtracking coils are disposed substantially coaxially at an angularinterval of substantially 90° around said sliding shaft.
 6. An objectivelens driving device comprising: a movable unit having an objective lensfor focusing an optical beam onto an optical disc; a focusing coil fordriving said objective lens in an optical axis direction thereof;tracking coils for driving said objective lens in a radial direction ofthe optical disc and a lens holder for holding said objective lens, saidfocusing coil, and said tracking coils; a sliding shaft which isprovided substantially in parallel to the optical axis direction of saidobjective lens and adapted to support and guide said movable unit; amagnetic circuit for generating magnetic flux which is applied to saidfocusing coil and said tracking coils; and a mirror for converting theoptical beam in the optical axis direction of said objective lens;wherein each of said tracking coils is disposed outside said objectivelens in the radial direction with respect to said sliding shaft, thedimension of each of said tracking coils in the optical direction ofsaid objective lens is set to be larger than the dimension of saidfocusing coil in the optical axis direction of said objective lens, andthe whole or a part of the optical beam incident to said mirror from thedirection which is substantially perpendicular to the optical axisdirection of said objective lens passes through a space within thedimension of said tracking coil in the optical axis direction of saidobjective lens.
 7. An objective lens driving device comprising: aplurality of objective lenses, corresponding to a plurality of differentoptical discs which are different in recording density or substratethickness, for converging an optical beam; a lens holder for holdingsaid objective lenses; a focusing coil and tracking coils which areprovided in said lens holder and adapted to drive said objective lensesin a focusing direction and a tracking direction; a sliding shaft forguiding said lens holder slidably and rotatably; a magnetic circuitwhich includes yokes and plural magnets and has a magnetic gap in whichsaid focusing coil and said tracking coils are inserted; wherein saidlens holder is provided with a magnetic substance which is disposed andformed radially with said sliding shaft at the center, and said magneticsubstance is disposed within a plane perpendicular to the planeconstituting said magnetic gap and outside said magnetic gap.
 8. Theobjective lens driving device as claimed in claim 7, wherein saidmagnetic substance is disposed substantially symmetrically with respectto the shaft center of said sliding shaft.
 9. The objective lens drivingdevice as claimed in claim 7, wherein said magnetic substance is formedof a plurality of rectangular plates.
 10. The objective lens drivingdevice as claimed in claim 7, wherein said magnetic substance issubstantially cross-shaped.
 11. The objective lens driving device asclaimed in claim 10, wherein said magnetic substance is provided with anotch portion through which an optical beam incident to said objectivelens passes.
 12. The objective lens driving device as claimed in claim11, wherein said notch portion is provided at a symmetrical position atthe opposite side with respect to the shaft center of said slidingshaft.
 13. The objective lens driving device as claimed in claim 7,wherein said magnetic circuit comprises said magnets, outer yokes towhich said magnets are secured, and inner yokes which confront saidmagnets through the magnetic gap, wherein the dimension of said inneryokes in the direction parallel to the optical axis direction of saidobjective lens is set to be smaller than the dimension of said magnetsin the direction parallel to the optical axis direction of saidobjective lens.
 14. The objective lens driving device as claimed inclaim 13, wherein each of said inner yokes is provided with a notchportion through which an optical beam incident to said objective lenspasses, and the end portion thereof confronting each of said magneticsubstances is provided with a round portion at at least one positionthereof.
 15. The objective lens driving device as claimed in claim 14,wherein the radius of said round portion is set to 0 mm to 2 mm.
 16. Anoptical disc apparatus having the objective lens driving device asclaimed in claim 7, and a control device for controlling the switchingoperation of said objective lenses by applying current to said trackingcoils in accordance with the type of optical disc, whereby plural typesof optical disc can be reproduced.
 17. An objective lens driving devicecomprising: a plurality of objective lenses corresponding to a pluralityof types of optical discs which are different in recording density orsubstrate thickness, for converging an optical beam; a lens holder forholding said objective lenses; a focusing coil which is provided in saidlens holder and adapted to drive said objective lenses in a focusingdirection; tracking coils which are provided to said lens holder andadapted to drive said objective lenses in a tracking direction; asliding shaft for guiding said lens holder slidably and rotatably; yokesand plural magnets which form a magnetic circuit having a magnetic gapin which said focusing coil and said tracking coils are inserted;further including magnetic substances which are radially disposed orformed on said lens holder with said sliding shaft at the center,wherein the arrangement angle of said plurality of objective lensesaround the shaft center of said sliding shaft is set to be differentfrom the arrangement angle of said magnetic substances around the shaftcenter of said sliding shaft.
 18. The objective lens driving device asclaimed in claim 17, wherein the arrangement angle of said plurality ofobjective lenses around the shaft center of said sliding shaft is set tobe smaller than the arrangement angle of said magnetic substances aroundthe shaft center of said sliding shaft.
 19. An objective lens drivingdevice comprising: a plurality of objective lenses corresponding to aplurality of types of optical discs which are different in recordingdensity or substrate thickness, for converging an optical beam; a lensholder for holding said objective lenses; a focusing coil which isprovided in said lens holder and adapted to drive said objective lensesin a focusing direction; tracking coils which are provided in said lensholder and adapted to drive said objective lenses in a trackingdirection; a sliding shaft for guiding said lens holder slidably androtatably; yokes and a plurality of magnets which form a magneticcircuit having a magnetic gap in which said focusing coil and saidtracking coils are inserted, further including magnetic substances whichare radially disposed or formed on said lens holder with said slidingshaft at the center; wherein the arrangement angle of said plurality ofobjective lenses around the shaft center of said sliding shaft is set tobe different from the arrangement angle of said magnets around the shaftcenter of said sliding shaft.
 20. The objective lens driving device asclaimed in claim 19, wherein the arrangement angle of said pluralobjective lens around the shaft center of said sliding shaft is set tobe smaller than the arrangement angle of said magnets around the shaftcenter of said sliding shaft.